The hydraulic system of a machine, such as, for example, an excavator or a loader, typically includes a pump and a hydraulic actuator in fluid communication. The hydraulic actuator may be a hydraulic cylinder, a hydraulic motor, or another device supplying motive power to a work implement or drive train of the machine. During the operation of the machine, pressurized hydraulic fluid flows from the pump to the hydraulic actuator to move a work element associated with the hydraulic actuator.
A pump generally includes a drive shaft, a rotatable cylinder barrel having multiple piston bores, pistons held against a tiltable swashplate, and a valve plate. When the swashplate is tilted relative to the longitudinal axis of the drive shaft, the pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel.
In the above described pump, the total fluid flow from the pump is geometrically proportional to the sum of the displacement of the individual pistons between bottom dead center (BDC) and top dead center (TDC) positions of the pump. A pump generally has an odd number of pistons and piston bores in the cylinder barrel. When the pump has, for example, nine pistons and corresponding pistons bores, there may be five pistons pressurized at a certain rotational position of the cylinder barrel and four pistons pressurized at another rotational position. This difference in the number of the pressurized pistons in a revolution of the cylinder barrel results in flow and pressure variations in the fluid output of the pump.
The flow and pressure variations frequently create pump noise, also known as a ripple. The ripple becomes more prominent as pressure variation amplitude and frequency increase. Such pump-produced variations or ripples in pressure and flow are transmitted through the hydraulic fluid as fluid-borne noise to the hydraulic actuator and other components in the machine. The fluid-borne noise in turn becomes audible (air-borne) noise and is transmitted to the surrounding air as undesirable noise and vibrations. Moreover, the ripple can exert a stress on the hydraulic actuator and other components in the machine, thereby decreasing machine life.
These flow and pressure variations are not limited to pumps having an odd number of pistons. In a pump having an even number of pistons, the numbers of pressurized pistons also change as the barrel rotates, and this also results in flow and pressure variations. In addition to the above described causes of flow/pressure ripple, minor geometrical changes and port timing can contribute to flow and pressure variations. Thus, the pump structure, pumping frequency, harmonics, and other factors may create flow and pressure variations in the fluid transmitted from the pump to the hydraulic actuator.
Various attempts have been made to reduce noise in hydraulic systems. For example, U.S. Pat. No. 5,492,451 discloses an apparatus and method for attenuation of fluid-borne noise in a hydraulic system. The apparatus includes a mechanism for sensing a flow ripple produced by a pump and a negative flow ripple generator for reducing or eliminating the ripple. The negative flow ripple generator provides a corrective flow to the hydraulic system to cancel the flow ripple. The negative flow ripple generator uses a piston and a solid state motor to create a negative ripple and does not use pressurized fluid from the main system pump.
Also, U.S. Pat. No. 6,234,758 discloses a hydraulic noise reduction assembly having a variable volume side branch in a hydraulic system. The variable side branch includes a variable fluid container operable to change its volume based on a pump speed. A controller receives a pump speed signal and outputs a signal to vary the volume of the fluid container to attenuate fluid noise in the hydraulic system. To attenuate fluid noise with low frequency, the hydraulic noise reduction assembly may require a fluid container with a large volume capacity.
Thus, it is desirable to provide a system that effectively attenuates fluid-borne noise in a hydraulic system, is relatively inexpensive to manufacture, and is compact in size. The present invention is directed to solving one or more of the shortcomings associated with prior art designs.